Along with technical progress in an image forming apparatus such as a copying machine, requirements for an image formed by such an image forming apparatus have been increasing. In response to such a growth in the needs of users, there is a commercially available image forming apparatus having developing substances of an increased number of colors and using an electrophotographic method, which is in contrast to a conventional image forming apparatus which forms an image using developing substances of four colors (Y (yellow), M (magenta), C (cyan), and Bk (black)). These apparatuses include ones which handle special colors such as red, blue, green, gold, silver, and fluorescent colors in addition to four common conventional colors of cyan, magenta, yellow, and black, ones of inkjet type which additionally use light cyan, light magenta, and the like, and various others. The use of developing substances of multiple colors makes it possible to reproduce more natural colors than those of an image formed by a conventional image forming apparatus.
As an image forming apparatus of this type, there is commonly used one which uses six developing substances including cyan, magenta, yellow, and black developing substances, and light cyan and light magenta developing substances containing pigments which have spectral characteristics equivalent to those of pigments contained in the cyan and magenta developing substances and are smaller in amount. In the following explanation, the cyan and magenta developing substances will be referred to as dark toners while the light cyan and magenta developing substances will be referred to as light toners.
FIG. 10 is a graph for explaining the density of input image data (density of image data), the density of a printed image (optical density), and the amount of adhering toner.
In FIG. 10, the characteristics indicated by solid lines show the adhesion amounts of the dark toners and light toners on a recording sheet with respect to an image density in an image forming apparatus using the dark toners and light toners. A characteristic indicated by a straight line 1000 is the characteristic of an ideal optical density with respect to the density of image data.
The adhesion amounts of the light and dark toners with respect to the image density are determined such that the optical density of an image formed using the light and dark toners is plotted to be ideally linear. As shown in FIG. 10, in an area extending from a low-density portion (highlight portion) to a medium-density area with a density of “0.9” or less, an image is formed using only the light toners in order to reduce the granulated effect of the image. In an area extending from the medium-density area to a high-density area, the dark toners are additionally used to suppress the amount of toner applied, and an image is formed using the light and dark toners.
In such an apparatus, the characteristic of an optical density varies depending on the environment and conditions in which the apparatus operates. For example, if the amounts of adhering dark toners have a characteristic as indicated by a curved line 1002 in FIG. 10, the optical density characteristic is plotted as indicated by a curved line 1001, and the optical density rapidly changes in the medium-density area where the dark toners start to be used in image forming. Accordingly, in an image containing a medium-density area, the tone may be unnatural or a false outline may occur.
For this reason, a variation in tone in a medium-density area is prevented by performing tone correction (tone correction for the colors of the light and dark toners) at power-on, at regular time intervals, at a predetermined time in a case where a variation in tone occurs, or in accordance with a user's instruction (see, e.g., Japanese Patent Laid-Open No. 2004-145137).
In such tone correction, a test pattern is formed on a recording paper sheet using developing substances (toners) that are used for image forming, and the density of the formed test pattern is detected by a sensor. The density is compared with that of original pattern data, and the tone correction is performed for a formed image. However, an image forming apparatus which adopts the tone correction using the light and dark toners requires formation of three types of test patterns, i.e., a test pattern formed using only the dark toners, a test pattern formed using only the light toners, and a test pattern formed using both of the dark toners and light toners. This causes a problem in that tone correction prolongs the downtime of the apparatus and reduces the productivity.